<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>五角星</title>
</head>
<body>


<canvas id="canvas" style="border:1px solid #aaa; display:block; margin:50px auto;">
	当前浏览器不支持canvas, 请更换浏览器后再试
</canvas>
<script>
	window.onload = function(){
		var canvas = document.getElementById("canvas");
		
		canvas.width = 800;
		canvas.height = 800;
		
		var context = canvas.getContext("2d");
		
		context.lineWidth = 10;
		drawStar( context, 150, 300, 400, 400);

	}
	
	/*
		正玄sin cos
	
	*/
	function drawStar( cxt, r, R, x, y ){
			cxt.beginPath();
			for( var i = 0; i < 5; i++){
				cxt.lineTo(Math.cos( (18 + i*72)/180 * Math.PI) * R +x,
								-Math.sin( (18 + i*72)/180 * Math.PI) * R + y);
				cxt.lineTo(Math.cos( (54 + i*72)/180 * Math.PI) * r + x,
								-Math.sin( (54 + i*72)/180 * Math.PI) * r + y);
			}
			
			cxt.closePath();
			cxt.stroke();
	}
	
</script>
</body>
</html>
